1. Field of the Invention
The present invention relates to a method of diagnosing presence of a fault in a main relay used for controlling supply of electric power to electric loads such as a microcomputer of an electronic control unit and various actuators mounted on a vehicle, and relates to an electronic control unit having a function of performing a fault diagnosis on such a main relay.
2. Description of Related Art
Most engine systems mounted on vehicles are provided with a self-diagnosis function. Such engine systems are configured to perform a self-diagnosis during a period when a vehicle engine is in a state where there is no fear of making a wrong diagnosis.
However, it often occurs that the time during which the vehicle engine is kept at the state where there is no fear of making a wrong diagnosis is too short to perform the self-diagnosis, depending on the contents of the self-diagnosis (diagnosis targets). One of such diagnosis targets is the evapo-purge system described, for example, in Japanese Patent Application Laid-open No. 8-35452.
The evapo-purge system is a system for collecting fuel steam evaporating from a fuel tank into a canister, and purging the collected fuel steam into an air intake passage of a vehicle engine together with fresh air sucked through an atmospheric hole of the canister depending on a running state of the vehicle engine. The evapo-purge system is mainly constituted by the canister, an evapo-passage for communication between the fuel tank and the canister, and a purge passage for communication between the canister and the air intake passage. To diagnose the evapo-purge system, the system is set in a closed state, and then a certain pressure is applied to the inside of this system. Abnormality in the evapo-purge system, that is, leakage due to a hole or a crack (mainly in the evapo-passage or the purge passage) can be detected by monitoring how the pressure inside the system varies when the application of the pressure to the inside of this system is removed.
If the fuel in the fuel tank is vibrated, or if the temperature of the fuel changes while the self-diagnosis is performed, the abnormality detection reliability may be degraded, because the fuel vibration and the fuel temperature change have a nonnegligible effect on the pressure inside the system.
Accordingly, it is known to configure an electronic control unit having a function of performing a self-diagnosis such that it automatically restarts in order to perform the self-diagnosis after a lapse of a predetermined time from the time when an ignition switch was turned off to stop a vehicle engine (refer to Japanese Patent Application Laid-open No. 2003-254172, for example).
Such an electronic control unit includes therein a soak timer. When the soak timer in which a predetermined timer time (start-up set time) is set is timed-up, a main relay is energized (turned on), as a result of which electric power is supplied to a microcomputer of the electronic control unit so that the electronic control unit can perform the self-diagnosis.
In such an electronic control unit, if the main relay develops a fault in that the main relay is always in the on state due to abnormality in a relay drive circuit, or the soak timer, or the main relay itself, it may result that the remaining capacity of a vehicle battery keeps decreasing and runs out at the worst time, because the supply of electric power to the electronic control unit continues.